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A thoroughly sporadic column from astronomer Mike Brown on space and science, planets and dwarf planets, the sun, the moon, the stars, and the joys and frustrations of search, discovery, and life. With a family in tow. Or towing. Or perhaps in mutual orbit.

When we moved into our house more than 7 years ago now the old owners left their Dish Network satellite TV dish attached to the roof. A few months later we got a sternly worded letter from the Disk Network demanding that we send them the dish back. With my detailed knowledge of the intricacies of the American legal system my obvious response was: come and get it. Which would have been fine with me. But, actually, that was not even my response, my response was to throw the letter in the trash while thinking in my head "come and get it."

Seven year later the dish was still on the side of the house. Luckily it is on the side that I never really see, so I didn't worry about it, but every now and then I thought to myself: "I should at least go up and take down that eyesore." But I never did. Until now.

I occurred to me a while ago that a parabolic dish like that would make a fine radio telescope (OK, it will end up a microwave telescope, but we'll get into the details later).

I'm not a radio astronomer or an electrical engineer or a Ham radio guy or any of that stuff, so I really had no idea what I was talking about, but it seemed a fun project for Lilah and I to play around with for the summer and for both of us to learn a little bit about microwaves. The caveat, though, is that my electronic explanations might not be exactly right. And I might break things.

We started last week. Step 1: remove the dish from the roof and see what was there. I had to snip the coax cables that went into the house and then undo five big screws and then everything just came unceremoniously down. The main issue was figuring out how to hold the wrench, dish, and ladder at the same time without falling. Luckily I survived this crucial part. Lilah stayed far enough away to avoid getting a dish on her head but to be able to both take pictures of me and make fun of me each time I dropped something and had to go pick it up.

The next step was to figure out what I really had. The dish itself is just a big semi-parabolic shaped piece of metal that focuses all of the microwaves into one spot. I wanted to see what was at the spot where the microwaves converge. Something, somewhere has to take the microwaves and turn them into an electrical signal that can travel down the coaxial cable. Given that coaxial cables come out the bottom of the arm, it must be that the conversion all happens right there on the end of the arm.

Here's the thing at the end of the arm. If you take the white plastic cap off (forgot to take picture; sorry!) you're looking into two things that look like funnels. I recognize those; they are "feedhorns" which really are, essentially, the funnels that will take the focused microwaves and transmit them somewhere, presumably to something inside that grey box. The grey box has, as outputs, three coaxial cables. It seemed obvious that the two feedhorns look in two slightly different directions, allowing a single dish to actually look at two TV satellites at the same time. In fact, they are labeled "119 degrees" and "110 degrees" so they presumably look at two satellite 9 degrees apart (verified. here are the TV shows you can watch on them.) One of the satellites, apparently, transmit microwaves that are polarized both up-and-down and also back-and-forth, meaning that you get twice as much TV from one satellite. The other satellite looks like it doesn't. Why not? I have no idea. But it does look like Satellite 119 (which two polarizations) is called the Main Satellite by the Dish Network and has more channels. So at least that makes sense.

The gray box is clearly the heart of the microwave receiver. From the label, it is a Digital LNBF. As I mentioned, I don't know any of this stuff. But, a judicious use of Google taught me that it stands for Low Noise Block Downconverter with Feedhorn. We've already found the feedhorn part, that just channels the microwaves into the box, so somewhere in there there has to be an antenna which actually detects the microwaves, not just moves them around, like all we've done so far.

I really wanted to see the antenna (three of them, actually, one for each polarization). So we started taking the box apart.

Sadly, most of the electronics are hard to get to and we never found the antenna. I can show you, however, what the inside of someone else's LNBF looks like. From Wikipedia, here is the LNB part of an LNBF. Imagine that the feedhorn is pointed straight at you and the microwaves are going into the feedhorn and into that empty circle in the upper middle. See the two thick wires sticking out, one horizontal and one vertical? Those are the antennae. The vertical ones will detect microwaves that are vertically polarized, the horizontal ones will detect the horizontally polarized microwaves. Just like you can use polarized sunglasses to remove one polarization from sunlight, the two antennae each will detect only that single polarization and the other will be invisible to it. Again, it's almost like having two separate satellites in the same spot.

Ever thought much about how an antenna works? They're quite simple, really. Microwaves, radio waves, light waves, and all of the rest are traveling electric and magnetic fields. These are "microwaves" because the size of the wave -- the range of distance over which the electrical field goes from positive to negative -- is a few centimeters. Imagine now that little wire -- the antenna -- sitting inside this electric field. If the antenna is about the size of the electromagnetic wave, one end of the antenna will get a positive charge, the other end a negative charge. Current will flow. You have just detected the radio wave. What if, instead, your antenna is really small compared to the wave? The change in electrical field across your antenna is tiny, so only a tiny charge flows. Nothing really happens. What if, instead your antenna is big compared to the wave? In that case you'll have many alternating positive and negative fields, but they will mostly cancel. Bad news. You detected nothing. The moral of this story is that you want your antenna to be about the same size as your wave (there are many details I am skipping here). And notice, the picture above, the antennae look to be something like a centimeter. Perfect for detecting the microwaves we're after.

From detecting the microwaves to sending the signals out the coaxial cables at the bottom is where the magic occurs. We have just received electrical signals on the antenna, but we can't send those directly down the coaxial cable. Why not? My lack of electrical engineering knowledge hurts me here. I know that microwaves have too high a frequency (or too short a wavelength) to transmit on coaxial cables without dying out incredibly quickly. But, I am sad to admit, I can offer you no intuitive explanation of why. I'll work on it. But the bottom line is that to transmit on a relatively cheap and simple coaxial cable you need to lower the frequency. That's where the "downconverter" part comes in. All of those electronics you see in the box take the microwave frequency detected, subtract a fixed frequency just a little smaller, and leave only low frequency oscillations that can travel down the coax. I understand the principle, but I'm not so good on the details.

If you want to watch TV, then, just point at the satellite, focus the microwaves, send them through a feedhorn, convert the microwaves to lower frequencies, send them via coaxial cable to your satellite receiver, and watch all you want.

One small technical point that will matter in a few minutes is that the electric power needed to run the LNBF is sent up from your satellite receiver on the same coaxial cable. This matters to us, since we're not going to have a satellite receiver and will need to construct our own power source.

We now know how our radio telescope is going to work. We'll point at an object, focus the microwaves with our dish, collect them in the feedhorn, convert them to lower frequencies, and then detect them. For TV you have to receive and decode them, but for us, all we want to do is detect their intensity. Luckily, this is a problem that TV watchers often have, too. When they want to align their satellite dish they need to find the satellite. The easiest way is to insert a "satellite finder" between the dish and the TV which simply measures the total amount of microwave received. You then tweak your dish around until you get the maximum signal.

You can buy fancy ones of these for $100+, but, for fun, I thought I would try something cheap, so I found this one on Amazon for $7.91 (including free Prime shipping; whoo hoo!). It has two coax inputs. One goes to the receiver, one goes to the LNBF. There is a meter, a buzzer (very useful, it turns out; the Amazon page didn't even mention it!), and a gain knob.

Recall that the LNBF gets its power from the TV receiver. This thing also gets its power from the receiver. It's the only reason for the coax to the receiver, in fact. If I had a satellite TV receiver, I could probably have just hooked it up straight and start using the dish. But I don't. So I need to power it myself. How?

The specs on the satellite finder where nearly nonexistent, but on the back (which, by the way, was easy to pop off), you could read: "Power ~13-18 V"

I like the "~" which gave me some comfort that I might be able to just wire something up quickly and perhaps the circuit wouldn't complain too much. The easiest thing I could think of to get a voltage in that range was two 9V batteries wired in series. Is this a good idea? I have no idea. You could, alternatively wire up ~10 AA batteries (at 1.5 V each) in series. Is this better? I have no idea.

It seemed easiest to me to do the 2 9V batteries, so Lilah and I headed down to Radio Shack and, for ~$3, got a bag of 9V connectors.

We wanted two batteries in series, so I connected the red (+) wire of one to the black (-) wire of the other. I soldered them together, but, really, that's over kill, isn't it? Just twisting and wrapping in electrical tape would probably be fine. Now we just need to figure out where to put the power in.

When you pop off the back you see this:

It's pretty easy to figure out where the power needs to come from. The only point of the coax from the receiver is to deliver that ~13-18V, so if we deliver it by battery instead we should be in business. On a coaxial cable the outside is ground (-) and the inside wire is the power (+), so we need to hook up our batteries appropriately. It's pretty easy to see what is connected to the outside and inside of the coax.

I did a simple soldering job connecting the red wire of the battery pack to the thing labeled "power input" above and connecting the black wire to the thing labeled "ground". Soldering! The smell reminds me of my father. Until recently I had his old soldering gun, but it, too, finally stopped working. Lilah loves it, too. I'm not quite bold enough to let her solder herself, but I think the time is quickly approaching. We talk a lot about proper technique, heating the wire, not the solder, checking the connections afterward with the voltmeter. I think she's ready.

The final product looks just fine. Red wire to the middle pin, black wire to the outer ground. I drilled a hole in the plastic back to allow the wires to pass through, and we got out the batteries for the moment of truth. What would happen when we plugged the batteries in? We really weren't sure. Smoke? Flame? Probably not. Nothing? Hard to say. So we did it. And here's what happened: a light came on on the front side! I didn't realize it, but the meter was backlit and we had just turned on the back lighting. We were powered, there was no smoke, and nothing seemed to be overheating! If I turned up the gain know I could eventually even get to a point where the buzzer was buzzing. It was time to try our microwave telescope!

All we had to do at this point was to go outside, attach the coax cable coming from the LBNF (there are two, actually, since they were originally attached to the two polarizations of the 119 degree satellite. Things in the sky are in general not polarized so it doesn't really matter which I used). And starting trying to find something. It looks something like this. The blue rubber bands hold the two batteries in place on the back. Quite professional, I know:

We turned the gain knob down until no sounds were being made and then I had an idea. The feed horn itself is like a tiny telescope, right? The dish just is a way to feed it even more microwaves. But shouldn't the feed horn detect thermal emission from something about 100F? Like, say, my hand? And the answer is, yes. When I put my hand in front of the feed horn the satellite finder started buzzing slightly. Lilah and I both yelped. Regardless of whatever else was going on, our feedhorn had just detected something when I put my hand in front. She put hers in front. Same sound. We were ready for astronomy!

Lilah and I had been talking about radiowaves and microwaves and how they were related to radios and to light and to things in the sky, so I asked her: "Lilah, what do you think would be the brightest thing in the sky in the microwave." "Well, dud, Daddy, it will be the sun," she said. She was right (well, almost). Unfortunately, for our first day of radio telescoping, we hadn't yet built a mount for the dish, so we used me.

I looked up, pointed the dish in the direction of the sun, and..... nothing. I waved it around, up, down, back, forth..... Nothing.We retested our hands: good. I pointed at the side of the house: good. But up in the sky there was nothing to be seen or heard.

Curious, I pointed all around the sky. When I started scanning south, the buzzer started going crazy now and then. I had just found the geosynchronous satellites broadcasting in the microwave. In fact, I was pointing in more or less the direction that our own dish had been pointing before I had removed it from the side of the house. I might have found the 119 degree Dish network satellite, even.

Knowing that we were capable of finding something I tried even more systematically for the sun. With Lilah helping to line up, we pointed right at the sun and slowly spiraled out, until, all of the sudden BUUUUUZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ!!!!!! First light! We had detected something beyond earth orbit. When I looked where I thought we were pointing I realized the problem: the feedhorn was offset from the center so that the dish could look at two satellites at once. We will be addressing that difficulty later. But the exciting thing about how hard it was to find the sun is that it means our dish is quite directional. It must be precisely pointed, even at something as bright as the sun.

We listened to the buzz of the sun's microwave emission for a while, while bathing in the shorter wavelength light of the sun also, feeling very proud of ourselves:

This is just the start of our summer project. We have proved to ourselves that we can repurpose the TV dish to detect things other than TV satellites (and we can detect those!). There is so much more to do. Here are the first few tasks we have:

Build a mount. Really. I am not that steady. A good mount will allow us to really explore the sky and see what is there.

Connect it to the computer. Right now we have a meter and a buzzer, which is good to see if things are working, but not so good for more quantitative exploring. I think connecting the buzzer to the microphone input of a laptop and then analyzing the signals might be doable.

See what else is in the microwave sky. The moon seems a likely target, because it is big and moderately hot. Venus? It's hot, but not so big, so I don't know. Other things? We'll found out.

Our telescope is crude, but I think both Lilah and I learned some stuff we didn't know before, which is always the goal. Plus we will have more of the summer to see what we can really find out there. We spent $7 on the satellite finder, $3 on the battery connectors, and got a dish for free. Looking at ebay it appears you could get a dish+LBNF for under $100. Go out and explore the microwave sky!

57 comments:

1) 10 1.5 AA batteries have more energy in them than 2 9V batteries. Either would work. You probably have a "12V" wall power supply lying around some place. These often generate 13V+.2) The LNBF solves a couple of problems. First it amplifies very low power high frequency signal so you can get it from the dish to you house (otherwise you would have a very small signal by the time you got to the house). Second it down converts the signal from 12 GHz to something that can smaller so that you can use inexpensive 50 ohm coax.3) Why the down conversion? We have two area of losses in a Coax cable: Skin effect & Dialectric loss. At high frequencies signals tend to travel closer to the surface of a conductor. Generally the resistance of the conductor increases with the square root of the frequency. Dialectric losses look roughly like a capacitor between the inner conductor and the outer shield. As the frequencies increase they tend to "short" across the dialectric. You can get around this but it is expensive.

Hmmm, so I worried about using a power supply for fear of RF noise. I considered an RF choke in the middle but decided against it out of laziness and lack of one sitting in my garage. Probably the satellite finder is sufficiently crappy that it makes no difference, though.

Failed to find the moon tonight, but we still have pointing issues. The mount appears unstable. Especially after a few glasses of wine.....

You absolutely take repurposing to a new and more interesting height! It isn't quite enough to make me want to get Dish service (so that I can then cancel it) but I appreciate the story and applaud the results!

Such that idea to make radiotelescope in 10GHz band from old parts I have got too,..because I worked with such antenas, convertors, splitters for that band. But what we can see on sky with 80cm antena, 0,3dB noise figure convertor, signal meter...? There are many geostationary telecom. satellites,...espec. over Europa we have lot of them. There are also terrestrial broad band TV, telecom.,..tranceivers. We can see also Sun,...Pavel

If you'd like to introduce Lilah to soldering someday soon, you might want to consider SparkFun. They sell electronic components to hobbyists, and they have some excellent through-hole kits that must be soldered together before you can use them. If you happen to be near Boulder, CO, they also teach classed.

This sounds like a project one would see demonstrated on the show "Create." However, if I were to repurpose a satellite dish, I would not just reverse engineer the device, but use the Internet to learn how satellite dishes work.

Once familiar with the workings of the device, I would look over the dish housing for a patent number. With the patent number(s) in hand, I would pay a visit to the U.S. Patent and Trademark Office web site (uspto.gov) and do a patent search on the patent number taken from the contraption. Study the patented technical plans for the dish and plan the "modifications" necessary to convert the satellite TV service dish into a telescope.

The amateur astronomy world has been wrestling with making cheap but functional pointing/mounting hardware for several decades. A DIY plywood Alt-Az. handbook is “The Dobsonian Telescope” by Kriege & Berry. This will let you point the beast precisely.

To counter the rotation of the earth a plywood equatorial platform will do so . My summer project is to assemble one using the wheels from a roller blade skate as the bearings.

There’s lots of help with both of these topics on the web.Time passes quickly! Enjoy your summer! George P.

@George: Good thoughts; thanks! I will probably just go alt-az and do driftscans to see what I detect. The dish mount already has an az adjustment that might work. So just some sort of turntable might do the trick.

Where I studied electrical engineering, they had a 2.5 meter parabolic antenna in the "backyard". At that time (about 1995) I was into doing some "Sat-D"X, where you point your satellite dish at geostationary satellites and see what TV feeds you can pick up. I imagined that they would do something similar. "Oh no, we do radio astronomy with it," the professor said. "Even cooler!" I thought to myself. But then he said that they can pick up the Sun and very faintly can pick up the Moon, and that was it. It was a bit disappointing, to say the least. For Sat-DXing, a 2.5 meter dish would have been simply "sweet" (or you know what a 2.5 Meter optical telescope can do), for astronomy it is merely a proof of concept, I'm afraid.

Or maybe the next time a big GRB passes us by, you could point it into the direction and hear something, but you would need to have a good alt-az mount for that – and constantly monitoring the alerts for such occurrences as GRBs and somesuch.

Or maybe one can use it to monitor the Sun, record how it changes in its 11 year cycle.

(If I remember correctly, the LNBs they had at my school worked at 4 GHz, and maybe with an 12 GHz LNB you can pick up more – maybe Jupiter or Saturn? – but I remain doubtful that it more than a crude tool to look at the Sun.)

Oh, by the way: There are all kinds of motors and mounts available for satellite dishes (at least here in Europe there is quite an range of different models). Some are alt-az two motor drives (even for motorhomes while on the road!), but most are nice classic equatorial mounts, but without the counterweight and only a RA drive motor. And as the needed pointing precision is much less than for optical astronomy, sat-mounts are not that expensive. Maybe one can even pick up a used motorized mount for next to nothing?

A propos telescopes: I think there hasn't been a detection of a trans-neptunian object >1000 km for some time, although the already found ones were thought to be members of larger populations. Or do I just misremember that? If not, is that more likely to be because those objects are rarer than was thought, or because the objects are there, but too slow and faint to be automatically detected?

I really like how simply you explained the process. I have studied radio astronomy before and it seemed like its all directed toward experts in field already. I'm still fuzzy on the band and frequency deal. If its all microwave radiation, and in radio astronomy you are simply detecting the changes in magnitude, then why would you want to scan for UHF VHF FM dB and certain channels on each?

Because in our case from planet Earth intelligent life is transmitting UHF/VHF/FM so if you wanted to search for intelligent life elsewhere in the solar system that could be transmitting radio waves to communicate (or beyond within tens of light years) then you might want to search these frequencies? They don't have the same kind of change in magnitude as celestial objects. That's my guess or to maybe go back and listen to old radio programs we transmitted billions of miles into space back in the 1950's? Assuming they might be able to bounce back off of super giant black holes or circle the universe back to Earth?

And while I still remain doubtful about DIY radio astronomy for celestial or solar system objects, here is a neat description of someone using a DIY modification of a existing maritime dish system to track man-made objects in LEO:

Basically with a KU-Band LNB (12 GHz) and any dish smaller than several meters (first link from above, "Tutorials and Fundamentals of Radio Astronomy"): Sun is easy, Moon is quit hard, everything else is impossible. Using a KA-Band LNB (at 20 GigaHz) might make the reception of the Moon much easier and it bring possibly into the reach of a 60 cm antenna (give or take).

The exception might be Cassiopeia A and Cygnus A, which should be detectable at 20 MegaHz with a larger (two to three meter) C-Band antenna (typically the old wire-mesh antennas from the era before direct TV) or a classic (non-parabolic) 20 MHz antenna.

If you go to higher frequencies than 20 GHz (if you can get such hardware that is) and look for specific things, you may (or may not) find a thing or two with smallish antennas (though I doubt it):http://www.setileague.org/articles/protectd.htmhttp://www.ukaranet.org.uk/basics/frequency_allocation.htm

This is simply brilliant!!! And funny too because a few years ago an amateur astronomer friend of mine put the thought in my head that if we could find a satellite dish we could "do" radioastronomy! I've thought about it over the years but dishes don't just appear out of the blue... Or so I thought! Last Wednesday evening a friend and I were walking our dogs when suddenly, there on the sidewalk, near the curb, in a beam of golden light was a treasure: a gorgeous satellite dish!!! I heard organ music, I swear I did!!! Oh my God! With beads of sweat appearing on my upper lip and my eyes popping out of their sockets, I gasped and wondered out loud if it was one of those "freebies" people put near curbs, but I was afraid to hope for too much... My friend only frowned at me in a peculiar way and herself wondered out loud too... not for the first time... about my sanity... Well sure enough, the nice owner of the house said yes, it's free!!! I returned later with my vehicle and now my amazing treasure is waiting for me to learn the next steps, so thank you Mike for this article and everyone for the useful comments!!! I can't wait to "see" the sky in different wavelengths! Yum!

An El Cheapo mount can be made out of a piece of 1.25" PVC pipe, an adapter fitting for that pipe to threaded pipe, a (threaded) pipe flange, and a 2' square of plywood (say, 1/2" thick).

Assemble the parts in order, and you'll have a portable post that you can stick the dish on, much the way it was mounted on your house. (I can send you a photo, if need be.) Turn the dish on the post for horizontal; loosen the dish's altitude adjustment bolt and turn the dish for vertical adjustments. Crude, but better than handheld.

"Clever" would be getting that same piece of PVC pipe, adding a 45 degree elbow and another foot or so of pipe. Adjust the dish mount to "45 degrees elevation", and mount it on the foot-long section of pipe so the dish looks parallel to the pipe. (The 45 deg. of the elbow plus the 45 deg. angle in the dish makes a 90 deg. change of aiming angle.) This is "clever", because now your dish's "handle" is a cylinder aimed in the same direction - not unlike a conventional telescope! Find a telescope mount that'll take a 1.25" telescope tube, and there you go. If you need a counterweight, a soda-pop bottle full of water or something works. Duct tape it to the other end of the pipe. Mount at the balance point.

Get a simple telescope mount, if you like; get a computer-controlled one if your wife / purse allows. This would let you track (say) Jupiter, even in the daytime! As for receiving a signal, you're most of the way there already.Reggie Arfordrarford # nb,net

Dear Mike,I am living in Iran and we have always problem for receiving satellite TV because of noises created by stupid Mullahs.Can we use your direction finder for detecting direction of noise?If we find correct direction we can adjust protecting metal nets around the dish accurately which provides us satellite TV channels without stupid noise!

The buzzer is probably just connected to a level detector - when the signal is above a certain level, it buzzes.

The meter, on the other hand, is probably connected to the output of a crystal detector; the same one that feeds the buzzer's level detector. At this stage, the audio is probably present on the "hot" side of the meter, at a very high impedance. Too high to drive a speaker. But, if you connect a buffer amp (like a big op-amp) to the "hot" side of the meter with an audio coupling capacitor (and of course the ground to ground on your circuit), the output of that should be enough to drive a small speaker.

The buzzer is probably just connected to a level detector - when the signal is above a certain level, it buzzes.

The meter, on the other hand, is probably connected to the output of a crystal detector; the same one that feeds the buzzer's level detector. At this stage, the audio is probably present on the "hot" side of the meter, at a very high impedance. Too high to drive a speaker. But, if you connect a buffer amp (like a big op-amp) to the "hot" side of the meter with an audio coupling capacitor (and of course the ground to ground on your circuit), the output of that should be enough to drive a small speaker.

I can't understand why, but when I plug in the coax to the 119 (or 110) the gain knob won't turn high enough for me to get a buzz... It did earlier in the day, but not in the later afternoon.Why? Is there not enough voltage?

"I'm having the same problem... It's weird, because it works at first but then goes lower and lower on the satellite reciever, and then I can't get any signal... Can somebody help PLEASE?"

That sounds exactly like the 9-volt battery you're using is almost dead. It's got enough power left to work for a little while, then it drains out. Next day, it's recovered a little; but drains out again.Replace the battery.

I suppose only those of you in the US might have heard this or read the article on the Web, but Dr. Gugliucci's sat dish looks to be the same model, and if I remember correctly, the Channel Master 1104IF was only about US$20 on eBay:http://www.sciencefriday.com/blogs/08/08/2014/build-your-own-radio-telescope-to-listen-to-meteors.html?series=33http://www.sciencefriday.com/blogs/08/08/2014/build-your-own-radio-telescope-to-listen-to-meteors.html?series=33

Moon can be detected, in theory, with small dishes, but the received signal is very close to the noise floor of the instrumet. The Moon albedo is only 0.12, so the ammount of solar energy reflected by the Moon is very small, and so the signal received by your equipment will be also very small.

The Small Radio Telescope was developed by Haystack Observatory to serve as an educational tool for universities and colleges for teaching astronomy and radio technology. http://www.haystack.mit.edu/edu/undergrad/srt/

This is amazing!! Great job of upcycling the old parts. It makes me smile knowing that you involved your kiddo so much. It is something that later in life, she will look back on and know she had a good attentive father.I do have a few questions. I know this is an old posting but if you are still checking the messages I do hope you find this one. Did you ever hook the telescope to a PC? If so by what means? Have you thought about a way to graphically plot the signals you have received to compare them to other signals?

I really like how simply you explained the process. I have studied radio astronomy before and it seemed like its all directed toward experts in field already. I'm still fuzzy on the band and frequency deal. If its all microwave radiation, and in radio astronomy you are simply detecting the changes in magnitude, then why would you want to scan for UHF VHF FM dB and certain channels on each?

Depends on the feed horn, circuitry in the LNB .... and lastly the receiver ...but the dish is just a reflector and will reflect most everything ... unless its a screen then -- as the frequency gets to a point that it passes in between the holes, the gain will go down.